An image forming device that has a duplex printing function was proposed in JP Laid-Open Publication No. 2002-167077. When the image forming device receives image data from a user terminal through a network, the image forming device determines whether or not the image data has instruction information for duplex printing first.
When the image data has the instruction information for duplex printing, the device turns on a thickness detection sensor located in a conveying path and drives a sheet feeding roller so that the device feeds a sheet stored in a sheet tray by rotation of the sheet feeding roller. The fed sheet is carried to the conveying path, and the thickness of the sheet is detected by the thickness detection sensor. A judgment unit judges whether the detected thickness is greater than or equal to the threshold value. The threshold value represents a predetermined thickness at or above which duplex printing cannot be performed.
The threshold value is set based on the following reasons. When a sheet on which a print process is performed on one side is thick and has large stiffness, a large curl occurs on the sheet because the sheet is carried through the conveying path while receiving a large bending force. When the sheet with the large curl is carried to the conveying path again to perform the print process on a reverse side, there is a high possibility that a paper jam will occur. Based on the reasons discussed above, the image forming device cannot perform duplex printing when the sheet thickness is greater than or equal to a predetermined threshold value in order to prevent the duplex printing process for a sheet that has an inappropriate thickness for duplex printing.
When the detected thickness of the sheet is less than the threshold value, the image forming device judges that the sheet is satisfactory for duplex printing and performs an image forming process and an image fusing process on a first side of the sheet first. The image forming device carries the sheet to a sheet inversion mechanism that is used at the time of the duplex printing process for turning the sheet with reverse-side up. Then, the image forming device performs the image forming process and the image fusing process on the reverse side, or second side, as was done on the first side and ejects the sheet from an outlet.
On the other hand, when the detected thickness is equal to or more than the threshold value, the image forming device judges that the sheet does not qualify for the duplex printing and drives a transferring belt and so on to carry the sheet, which is fed to a conveying path, by the belt rotation for ejecting the sheet from the outlet without printing.
When the image forming device performs the duplex printing process, and when the thickness of the sheet, which is fed from the sheet tray, is equal to or more than the threshold value, the print process is forcibly stopped as discussed above. Therefore, there is a problem that the device may fail to meet the user's needs.
In view of the problem mentioned above, an object of the present invention is to provide another option for a user through outputting selection information that is whether or not the duplex printing process is performed by using another sheet stored in another sheet tray and that is to be selected by the user when the thickness of the sheet fed from the sheet tray is equal to or more than the threshold value.